The effect of climate change on Arcto‐Tertiary Mexican beech forests: Exploring their past, present, and future distribution

Abstract Fagus mexicana Martínez (Mexican beech) is an endangered Arcto‐Tertiary Geoflora tree species that inhabit isolated and fragmented tropical montane cloud forests in eastern Mexico. Exploring past, present, and future climate change effects on the distribution of Mexican beech involves the study of spatial ecology and temporal patterns to develop conservation plans. These are key to understanding the niche conservatism of other forest communities with similar environmental requirements. For this study, we used species distribution models by combining occurrence records, to assess the distribution patterns and changes of the past (Last Glacial Maximum), present (1981–2010), and future (2040–2070) periods under two climate scenarios (SSP 3‐7.0 & SSP 5‐8.5). Next, we determined the habitat suitability and priority conservation areas of Mexican beech as associated with topography, land cover use, distance to the nearest town, and environmental variables. By considering the distribution of Mexican beech during different periods and under different climate scenarios, our study estimated that high‐impact areas of Mexican beech forests were restricted to specific areas of the Sierra Madre Oriental that constitute refugia from the Last Glacial Maximum. Regrettably, our results exhibited that Mexican beech distribution has decreased 71.3% since the Last Glacial Maximum and this trend will for the next 50 years, migrating to specific refugia at higher altitudes. This suggests that the states of Hidalgo, Veracruz, and Puebla will preserve the habitat suitability features as ecological refugia, related to high moisture and north‐facing slopes. For isolated and difficult‐to‐access areas, the proposed methods are powerful tools for relict‐tree species, which deserve further conservation.


| INTRODUC TI ON
The abrupt ice cover variation that occurred during Eocene-Oligocene (~33.7 Ma BP; Helmer et al., 2019) influenced the global climate by interrupting the cooling trend. During these periods, vast amounts of CO 2 were emitted into the atmosphere, and evaporation from the sea increased as reflected in benthic foraminifera values (from 1 to 3 oxygen isotope composition [δ 18 O 0/00 ]; ~300,000 years; Graham, 1976; Figure 1), influencing hydrological cycles (Tang et al., 2017). During this period, we might have expected an increase in cloudiness although extensive studies have shown a decreasing trend of minor cloud immersion affecting the tropical montane cloud forests (TMCFs) and thus triggering local extinctions via enhanced dryness (Ponce-Reyes et al., 2012). Identifying the extent of the relict-endangered plants' response to climate change helps design flexible conservation strategies for Mexican TMCFs. One of the most interesting TMCFs characteristics is their specific floristic diversity (with ~22,800 vascular plant species) and high endemism (Silveira et al., 2019).
Arcto-Tertiary Geoflora species (sensu Baskin & Baskin, 2016;Chaney, 1959) as currently distributed in the Mexican TMCFs include temperate tree genera with broad ecological tolerances (e.g., Carya, Fagus, Liquidambar, Liriodendron, Magnolia, Meliosma, and Tilia;Graham, 1976). Therefore, the Mexican TMCFs have had a stable long-term climate and remain in critical hotspots (known as Pleistocene glacial refugia; Rico et al., 2021) and possessed a high conservation priority for the long-term persistence of relict-forest communities (Tang et al., 2017). Before and during the Ice Ages F I G U R E 1 Paleo-distribution map of North American beech species. Light blue curve shows global average Δ 18 O derived from benthic foraminifera, which mirrors the major global temperature trends from Eocene to Quaternary Glacial (modified from Jiang et al., 2020)
In the Oligo-Miocene (c. 25 Ma BP), temperate tree genera such as Fagus appeared in the Mexican TMCF floristic composition (Graham, 1976 (Biaggi, 1978;Graham, 1976Graham, , 1999Palacios Chavez & Rzedowski, 1993). This is where the origin of "modern" terrestrial ecosystems has been documented, appearing in several of the world's hotspots of terrestrial biodiversity (Rahbek et al., 2019). Mexican beech is an unprotected species, although on the IUCN Red List it is cataloged as LC ("Least Concern," https://www.iucnr edlist. org/speci es/62004 694/62004 696#popul ation). The Mexican beech forests occur at altitudes of 1509-2034 m above sea level (asl) on northern steep-ravines with high moisture, on vitric slopes, and near streams harboring unique floristic assemblages with a specific microclimate .  Figure 2) and determined forest areas and environmental conditions for each state ( Table 1). The occurrence data for each selected stand were used to build the past, present, and future prediction models.

| Present climate
We used elevation data and 19 bioclimatic variables from the CHELSA database, which involved a recent time frame , the aridity index, and the annual evapotranspiration variables from the CGIAR-CSI website (www.cgiar -csi.org; Trabucco & Zomer, 2019), with a resolution layer of c. 1 km 2 . We selected bioclimatic variables for the Mexican beech based on predictive maps following two different approaches, one statistically based (Fielding & Bell, 1997;Peterson & Soberón, 2012) and the other using Mexican beech occurrence records (Ehnis, 1981;Pérez-Rodríguez, 1999;Rodríguez-Ramírez et al., 2013Rowden et al., 2004).
Lastly, we obtained climate data from the CHELSA database (Karger et al., 2022).
We selected two future climate scenarios (Shared Socioeconomic Pathways; SSP 3-7.0 and 5-8.5) under two greenhouse gas concentration regimes and without implementing future climate policies (e.g., Kyoto Protocol; Bosso et al., 2017;Xian et al., 2022). The SSP 5-8.5 scenario assumes high greenhouse gas concentrations throughout the 21st century, reaching equilibrium by 2100, whereas the SSP 3-7.0 scenario represents global greenhouse gas concentrations peaking in 2070. We assumed that SSP 5-8.5 would be the most chaotic scenario if we assume no measures are taken to avoid the climate effects; SSP 3-7.0 would be a less chaotic scenario, assuming emissions reductions occur. Finally, to assess the climate change effects on the Mexican beech distribution in TMCF of eastern Mexico, we developed an species distribution models (SDM) as a function of the environmental variables considered above in the five selected models, for the SSP 3-7.0 and SSP 5-8.5 scenarios.

| Species distribution modeling
We estimated the relationship between environmental variables and Mexican beech presence using Pearson correlations (r < .70), retaining those relationships. Additionally, we performed a Jackknife analysis that incorporated the following options: False discovery rate calculation, the multicollinearity degree, coefficient of determination of linear regression, tolerance, variance inflation factor, the Bayesian information criterion (BIC), and Akaike (AIC). Finally, we combined this with our knowledge of Mexican beech responses to specific environmental factors (i.e., mean annual temperature, mean temperature of the warmest quarter, mean temperature of the coldest quarter, annual precipitation, and altitude; Fang & Lechowicz, 2006;Peters, 1997). We performed the analysis using the software R v. 4.1.2 with fuzzySim and sdm packages (Barbosa, 2021;Naimi & Araújo, 2016).
Assessment of the best candidate model was performed using the KUENM package (Cobos et al., 2019). We derived the potential distribution from the best model, using the average performance evaluation indicators (AUC), partial ROC (receiver operating characteristic), omission rate, and the optimal complexity parameter (AIC-Akaike Information Criterion; Bozdogan, 1987;Elith & Leathwick, 2009;Gutiérrez et al., 2018). We followed a logistic threshold for training presence clipping which corresponds to the 10% of data with the lowest probability value which is commonly used in conservation studies (Abba et al., 2012;Ancillotto et al., 2019;Khanghah et al., 2022). In addition, we used the trimming threshold (~24%-26%) of the present model, which approximates the distribution of the species according to Williams-Linera et al. (2000) and Rodríguez-Ramírez et al. (2021). We then selected the models from the "best" variable set (500), scenario, and candidate model (Appendix S1).
We determined the surface in each climatic model (km 2 ), assessing the surface variation among past, present, and future scenarios (Appendix S2). Next, we overlayed the Mexican beech geographic models and then divided the probabilities of occurrence into five We performed a spatial distribution bias correction to avoid overadjusting future projections in the SDM. We included 10,000 bias files (points where the species is not recorded) and environmental variables to assess potential habitat suitability analysis. We achieved the analysis with the SDMtoolbox package in ArcGIS v. 10.8 (Brown et al., 2017). We implemented a Gaussian Kernel (Bosso et al., 2022;Mushtaq et al., 2021;Zhang et al., 2018) using QGIS software to avoid a sampling bias and help identify the highest potential suitability areas. With this, we selected the high suitable priority areas (hotspots) for conservation.
We projected suitable habitats for Mexican beech as expressed by the occurrence probability (AUC = 0.9993 ± 0.0614).
The Jackknife analysis detected that the mean annual temperature According to the current potential distribution, we identified that

| Mexican beech potential distribution under different future climate scenarios (2040-2070)
We and from 1400 to 2850 m asl for the SSP 5.8-5 scenario (Figure 3b).

| Priority conservation areas
The Mexican beech habitat suitability analysis detected (present  Table 3; Figure 4).
We detected hotspots outside of Natural Protected Areas, which represent 90.41% of the potential Mexican beech habitat suitability. Here, we detected specific environmental variables that influence habitat suitability which included: mean annual temperature (BIO1; from 6 to 18°C), annual precipitation (BIO12; from 1000 to 2450 mm), precipitation of coldest quarter (BIO19; from 75 to 190 mm), and evapotranspiration (from 1000 to 1650 mm/day).
These results indicate that Mexican beech occurs is comprised of moist environments (Cwb; Peel et al., 2007; Figure 4).

| DISCUSS ION
Patterns of species distribution reflect the interaction between many factors, such as microenvironmental, elevation, anthropic activities, intra-and interspecific relations, topography, and physiological features of the tropical species (Báez et al., 2022;Rahbek et al., 2019). In this comprehensive study, we considered specific environmental variables (Fang & Lechowicz, 2006 The above hypotheses suggest that the recent Mexican beech distribution existed before the climate slowly cooled toward a series of Ice Ages (Peters, 1997). Our models of past distribution exhibited that the species covered an area of 7388.51 km 2 , resulting from the reduced availability of suitable habitats brought about by climatic fluctuations. This interpretation is supported by palynological records (Biaggi, 1978;Graham, 1999;Palacios Chavez & Rzedowski, 1993). At present, we have detected an archipelagic distribution that maintains specific Mexican beech ecological refugia (e.g., Hidalgo, Veracruz and Puebla), which is supported by the suitability habitat analysis (Figure 4). According to the current Mexican beech forest coverage (1.647 km 2 , where it has become extinct (Quijano et al., 2016) validating the generated model (Figure 4). Therefore, it is possible that ecological niche conservatism has influenced the persistence of the Fagus species worldwide (Cai et al., 2021), which limits the distribution of ecologically dissimilar lineages among geographic regions (Jiang et al., 2020;Wiens & Graham, 2005). These regions have complex topographies and specific microclimate factors (e.g., fog, moisture, and mild temperature conditions; 14.8-15.6°C) throughout the Sierra Madre Oriental (Rodríguez-Ramírez et al., 2021).
Climate change represents a significant potential threat to the future existence of TMCFs (Los et al., 2021). Projected climatic conditions will cause an increase in temperature (4.1-5°C above current temperature) and CO 2 (70.04 gigatons for SSP 3-7.0 and 116.8 gigatons for SSP 5-8.5; Taylor et al., 2012).
Our results confirmed that the Mexican beech forests will likely reduce its range, based on our climate change projections through 2070 (>38% of its potential current extent), by more than 80% over the next 50 years. Mexican beech forests could prefer isolated mountainous regions of the Sierra Madre Oriental with more suitable moisture and temperature conditions (Cai et al., 2021;Fang & Lechowicz, 2006). The above ideas confirm the high climatic sensitivity of Mexican beech to climate change, in particular to drought periods, increasing its extinction risk Téllez-Valdés et al., 2006).

| Management and conservational implications
The Mexican beech presents preservation conflicts because of anthropic activities such as beechnut harvesting, grazing, illegal logging, and corn/avocado plantations (Rodríguez-Ramírez et al., 2013;Williams-Linera, 2007 Rico et al., 2021) from the perspective of autoecology, which is essential for management and conservation implications, with emphasis on the ecological refugia as was recorded in this study.

ACK N OWLED G M ENTS
The authors acknowledge support from the project DGAPA-PAPIIT IN220621. We are grateful to Numa P. Pavón for providing the first record of the Mexican beech in Huayacocotla, Veracruz; and Francisco Vega for the first record in Pahuatlán, Puebla.

CO N FLI C T O F I NTE R E S T
The authors declare no conflict of interest.